The invention concerns a roll used in the production of paper, in particular a center roll in a press section of a paper making machine, with which the web is in direct contact and from which the web is detached, a composite structure being formed on the cylinder mantle of the roll.
The invention also concerns a method for the manufacture of a roll in accordance with the invention.
As is well known, in the press section of a paper making machine, a rock roll is used, which is made of granite. The popularity of granite is based on its surface properties, which provide a controlled detaching of the paper web from the rock face. Moreover, granite withstands the wearing effect of a doctor well.
However, granite has certain drawbacks. Being a natural material, its properties vary, and internal flaws in granite and its tendency of cracking constitute a serious obstacle for its use in some applications. Moreover, a granite roll is heavy, which increases the tendency of vibration of the structure. The weight of the rock roll is also reflected in the dimensioning of the lifting equipment and of the foundations of the paper machine.
In the prior art, synthetic rock rolls are known, which are, in principle, polymer-faced rolls in which rock powder, such as quartz sand, has been added to hard rubber or polyurethane. Drawbacks of these rolls have been excessive adherence of the paper web to the roll face as well as poor mechanical strength.
In the Applicant's Finnish Patent No. 70,273 a press roll is described whose surface layer is composed of a mixture of metal powder and an inorganic substance. The function of the metal is to act as a binder agent and to increase the toughness of the roll coating. The function of the inorganic substance is to provide a wear-resistant face of suitable surface energy, because the surface energy of the roll face must be within certain limits in order that the detaching of the paper web from the face of the press roll can be controlled.
In a roll in accordance with the Applicant's Finnish Pat. Appl. No. 853544 a suitable surface energy has been even better achieved, such that the metal component is stainless steel expressly containing chromium, the proportion of chromium in the metal being 9 to 35%. A stainless steel that contains an abundance of chromium is a hydrophilic material (chromium increases the hydrophily). On the other hand, by means of the alloying of chromium, wear-resistant chromium carbides were obtained in the structure. Chromium also increases the resistance of steel to corrosion. In such an "alloy", the ceramic material is separated from the steel itself as a chromium carbide.
In the Applicant's Finnish Patent Application No. 882006 a solution is described whose primary objective is to provide a roll and a method for the manufacture of the roll by means of which the detaching of the paper web from the roll face is controlled and the resistance of the roll to temperature and to mechanical strains is increased. With a view to achieving this objective, the roll in accordance with the FI Pat. Appl. 882006 is characterized in that the outer face of the roll coating consists of carbide-rich areas and of matrix areas placed between said areas.